This invention relates to tracking error detection and, more particularly, to a method and apparatus for detecting a tracking error in a playback head which scans successive tracks across a record medium.
Tracking error control arrangements are known for controlling the position of one or more heads as those heads scan tracks previously recorded on a record medium. For example, in conventional video recording, video signals are recorded on a magnetic tape in successive slant tracks by a helical scan rotary head assembly. When the video signals are reproduced, either by the same head assembly or by different apparatus, the very same slant tracks that had been recorded must be followed accurately for proper signal reproduction and some form of tracking control generally is used for this purpose. Similar tracking control normally is provided when audio signals are recorded in slant tracks, such as in digital audio tape (DAT) recording.
In one type of tracking control arrangement, proper positioning of the heads with respect to the slant tracks is carried out with the assistance of a control signal recorded along a longitudinal edge of the magnetic tape. This longitudinally recorded control signal is reproduced by a stationary head; and tape movement is adjusted if there is a phase difference between the reproduced control signal and a position signal representing the rotary position of the heads. For example, proper positioning of a head relative to a track is indicated if the head position signal, which is produced when the head rotates to a predetermined position (e.g. to the beginning) of a track, coincides with a reproduced control signal. In the absence of coincidence, the tape is driven either faster or slower until the head is brought into proper alignment with a track, as represented by such coincidence.
A disadvantage of this conventional technique is the requirement of a stationary head which is dedicated to a single function. This head adds to the overall cost of the signal recording/playback apparatus and, more importantly, it impedes miniaturization of that apparatus. If the stationary head can be eliminated, the overall size of the apparatus can be reduced.
It has been proposed to mix tracking control signals with information signals in the same slant tracks. This has the advantage of eliminating the aforementioned stationary head and relies upon the transducer which reproduces useful information to reproduce the tracking control signals as well. Digital video and digital audio recording are particularly receptive to this approach because digital information signals are easily multiplexed with control signals without loss of useful information, they may be time base compressed easily and they may be readily processed. Hence, in a particular track, a tracking control signal may be recorded in a restricted region that normally is preceded and followed by digital information signals.
In one implementation of this proposal, a pilot signal of predetermined frequency is recorded in alternate tracks, and each pilot signal region is preceded and followed by digital signals. Advantageously, digital video and digital audio signals may be recorded in tracks having no guard bands between them. While this increases the amount of magnetic tape made available to record useful information, it also emphasizes the need for precise tracking control as playback heads scan the respective tracks. Typically, the heads used for recording and/or reproduction exhibit a gap length larger than the width of a track, which means that an edge of a preceding track is overwritten when the next adjacent track is recorded. This also means that when a playback head is in proper alignment with a prerecorded track, the signals recorded in both adjacent tracks are picked up as crosstalk components. This phenomenon of crosstalk pickup is used to detect tracking errors and to provide proper tracking control over the scanning heads.
In the arrangement mentioned above, wherein pilot signals of a predetermined frequency are recorded in restricted regions of alternate tracks, a pair of heads is used to scan two adjacent tracks substantially simultaneously. Hence, when one head scans the track in which the pilot signal is recorded, the other head scans the adjacent track which has no pilot signal therein. These two heads are mounted on the same rotary head assembly, such as a rotary head drum, and are relatively closely spaced thereon. The head which scans the track having no pilot signal therein will pick up, as a crosstalk component, the pilot signal recorded in an adjacent preceding track as well as the pilot signal recorded in an adjacent following track. If the magnitude of the crosstalk pilot signal picked up by this head from the preceding track is equal to the magnitude of the pilot signal picked up from the following track, the heads are in proper alignment with respect to the tracks scanned thereby. A tracking error is detected if the crosstalk pilot signal magnitudes from the preceding and following tracks differ from each other. In that event, the tape transport is accelerated or decelerated to bring the heads back into proper tracking alignment.
In the foregoing implementation, wherein pilot signals are recorded in restricted regions in alternate tracks, it will be appreciated that when a head scans a track having no pilot signal therein, the pilot signal recorded in the following track will be picked up as a crosstalk component before the pilot signal recorded in the preceding track is picked up. Assuming that the pilot signals are recorded in substantially equal regions in each track, it will be further appreciated that the pilot signals picked up as crosstalk components from the following and preceding tracks will be present concurrently for a significant period of time, and that the pilot signal picked up from the following track will terminate first. Thus, during the scan of a track in which no pilot signals are recorded, there will be distinct intervals of time during which the pilot signal is picked up as a crosstalk component from one adjacent track and then from the other. However, these time intervals are relatively short. Consequently, there is the possibility that the crosstalk pilot signal picked up from one of the adjacent tracks during such short interval may not be detected satisfactorily. Furthermore, this brief time interval is reduced if the track pitch is decreased; and a reduction in track pitch desirably leads to greater recording density. Consequently, if the recording medium is subjected to jitter, the time interval during which one crosstalk pilot signal is picked up may be so small as to inhibit adequate detection. Thus, a tracking error may not be properly sensed and tracking control is degraded.